As is well known, as represented by a glass substrate for a flat panel display (FPD) such as a liquid crystal display, a plasma display, or an organic light-emitting diode (OLED) display, thin glass sheets utilized in various fields are actually required to satisfy a rigorous product quality requirement for surface defects and waviness.
As a manufacturing method for a thin glass sheet of this kind, an overflow downdraw method may be utilized, which is capable of obtaining a glass surface which is smooth and free of defects.
As illustrated in FIG. 5, this manufacturing method includes: pouring molten glass G into an overflow trough 4 formed in a top portion of a forming member 1; allowing the molten glass G which is overflown to both sides from the overflow trough 4 to flow down along outer surface portions 5 (each including a perpendicular surface portion 5a and an inclined surface portion 5b) of the forming member 1, which have a substantially wedge shape; and fusing together streams of the molten glass at a lower end portion 5c of the forming member 1, thereby continuously forming a single thin glass sheet. This manufacturing method is characterized in that both front and back surfaces of the thin glass sheet are formed in a forming process without coming into contact with any area of the forming member 1, and hence a fire polished surface with extremely high flatness and smoothness and without defects such as flaws can be obtained.
Specifically, the forming member 1 to be used for this manufacturing method includes, for example, restricting wall portions 3b for restricting widthwise spread of the molten glass G flowing down along the outer surface portions 5 of a forming member main body 2 (see, for example, Patent Literature 1 and Patent Literature 2). In this case, both widthwise end portions of the molten glass G overflown from the overflow trough 4 are guided downward along the restricting wall portions 3b at a stage at which the molten glass G has reached the outer surface portions 5 of the forming member main body 2. As illustrated in FIG. 6, the restricting wall portions 3b are generally formed by fitting a pair of covering members 3 (specifically, fitting depressions 3a) onto both widthwise end portions of the forming member main body 2. To give an additional description, end surfaces of the covering members 3 fitted onto both the widthwise end portions of the forming member main body 2 serve as the restricting wall portions 3b. 